This invention relates to a device comprising a fastener formed with a male screw thread and which, in use, is subjected to tension. The invention may be applied to bolts and studs.
The invention concerns the provision in a fastener device of means for enabling a user to determine when the device has been tightened to a predetermined degree. An example of a fastener device having such means is disclosed in U.S. Pat. No. 2,413,797. This known fastener device is a bolt having a bore which extends along the axis of the bolt and is open at each end of the bolt. In an end face of the bolt remote from the head, there is provided a transverse groove. A rod extends along the entire length of the bore and is secured to the bolt adjacent to the head thereof. At the opposite end of the rod, there is provided an elongated head which, when the bolt is unstressed, protrudes from the end of the bolt. When the bolt is subjected to a predetermined stress, the bolt is stretched and the head of the rod seats in the transverse groove so that it no longer protrudes from the end of the bolt. This indicates visually to the user that the bolt has been tightened a predetermined amount.
The degree of accuracy with which a predetermined stress can be established in the bolt disclosed in the aforesaid U.S. Patent depends upon the quality of the eyesight of the user, qualities of the fastener device, for example the colour and cleanliness of the head of the rod and of the end of the bolt, an adequate level of illumination of the fastener device and upon whether the fastener device can be viewed from an appropriate position so that the line of sight is substantially perpendicular to the length of the bolt.